I. Field of the Invention
This invention relates generally to methods of manufacturing electronic circuit assemblies, and more particularly to a method of semiconductor packaging of single or multiple die using tape automated bonding tape for test and elevated temperature cycling (burn-in) of such semiconductor packages.
II. Discussion of the Prior Art
As integrated circuit (IC) chips are made smaller and of increased complexity and as clock speeds become higher and higher, methods had to be developed to provide a suitable interconnection between the IC chips and printed circuit substrates on which such IC chips are mounted. To address the complexity and size constraints imposed, electronic packaging engineers earlier devised a process called tape automated bonding, often referred to by the acronym TAB. Tape Automated Bonding (TAB) is an inner connect technology between integrated circuits and associated substrates wherein a prefabricated carrier with copper leads adapted to the IC pads instead of individual wires. This prefabricated carrier or tape consists of a perforated polyimide film, like camera film, and of the same dimensions, which has a transport perforation and stamped openings for the IC and the connection leads. A copper foil is glued to the film and the copper is etched using photolithography. TAB is an approach to reduce the pitch and to speed up interconnection of IC chips to a lead frame, especially for high-volume production applications. There are several types of copper/polyimide composite film materials that have been found quite suitable. The polyimide sheet material is laminated with the copper foil using a thermosetting adhesive. The copper is appropriately masked and etched using well-known techniques to define inner and outer pad areas connected by fine, closely spaced conductive traces. Vias through the thickness dimension of the tape are formed and the vias are appropriately metallized, such as by electroless copper plating or by a direct metallization process (DMP) which are the thru-hole copper plating processes that involve conductive thin film deposition on the organic polymer surface of the thru-hole wall and copper electroplating on it. The conductive traces on an undersurface of the TAB tape individually lead to a plurality of solder ball mounting pads commonly referred to as a ball grid array (BGA) that facilitate attachment of integrated circuit chips mounted on the TAB tape to a printed circuit substrate.
In accordance with the prior art and as reflected by the flow chart of FIG. 1, wire bonding techniques are used to connect a fine wire between an on-chip I/O pad and a pad on the TAB tape. Once all of the wire bonds are established, the semiconductor die or dies are overmolded with a plastic encapsulant, after which solder balls are attached to the pads in the BGA located on the undersurface of the TAB tape beneath the die.
Following the teachings of the prior art, the TAB tape is then severed about the perimeter of the integrated circuit to yield discrete chips. The discrete chips are then placed in a custom made ball grid socket so that testing and burn-in can be performed on the chip before it is mounted in the end device using a reflow solder technique to join the solder balls of the BGA to the printed circuit board of the end device.
BGA sockets used for test and burn-in are dependent on ball pitch, ball size and the package outline. Any variation from the standard results in an expensive, custom test socket. Burn-in of BGAs often results in deformation of the solder balls. In that BGA test sockets require a specific ball size, any change in ball size results in the need for retooling of the BGA sockets. This necessarily increases the manufacturing cost of an integrated circuit assembly.
In accordance with the present invention semiconductor I/O are routed on TAB tape first to BGA pads internal to the die outline and secondly to test pads located external to the semiconductor package outline. In this fashion, the test pads can be used during test and burn-in, making it unnecessary to have expensive BGA sockets in order to do the test and burn-in. The method of the present invention also eliminates the need to contact the solder balls on the BGA during testing and burn-in so that they remain properly spaced and shaped for later reflow solder attachment to mating pads on a printed circuit assembly.